1. Field of the Invention
This invention relates to a color image forming apparatus in electrophotography. More particularly, the present invention relates to a color image forming apparatus which is compact in size and has an improved maintenance property and improved performance for jam processing.
2. Description of the Prior Art
In an electrophotographic image forming apparatus for forming multi-color images such as full color images or functional color images, a charging device for charging a photosensitive drum as an image retainer, image exposure means for exposing images on the photosensitive drum, developing devices for developing an electrostatic image on the photosensitive drum with a toner, transfer device for transferring a toner image on the photosensitive drum onto recording paper, and the like, are disposed around the photosensitive drum.
Among them, the developing device in the electrophotographic image forming apparatus for the full color image consists of three or four developing devices for storing the toners of yellow (Y), Magenta (M) and cyan (C) and, in some cases, black (BK). Each toner image of the yellow (Y), Magenta (M) and cyan (C) and in some cases, black (BK), is formed on the photosensitive drum while it rotates three times or four times, and each toner image is transferred to recording paper to obtain the full color image. In such an image forming system, the color image forming apparatus can be made compact by reducing the width of the three or four developing devices and disposing them closely adjacent to one another.
In Japanese Patent Laid-Open No. 55661/1986, a color image forming apparatus in accordance with such an image forming system is proposed. Latent images are formed by three exposure means corresponding to three developing devices, respectively, and the photosensitive drum, the developing devices and cleaning means are integrated as one unit so that the unit can be taken out from an apparatus main body. On the other hand, Japanese Patent Application No. 320596/1987 discloses an apparatus in which one exposure means is used, developing devices are arranged in superposition with one another and integrated as one unit in the same way as described above so that the integrated unit can be taken out from the apparatus main body.
In the above-mentioned color image forming apparatuses described in Japanese Patent Laid-Open No. 55661/1986 and Japanese Patent Application No. 320596/1987, the maintenance property can be improved. It is true, but spill and false mixing of developers are likely to occur when the unit is fitted to, or removed from, the apparatus main body.
In color image forming apparatuses for transferring each color toner image to recording paper by use of a transfer drum, Japanese Patent Laid-Open No. 72159/1983 discloses an apparatus wherein devices around the photosensitive drum are integrated as one unit.
However, the color image forming apparatus disclosed in this Japanese Patent Laid-Open No. 72159/1983 is not free from the following drawbacks.
(1) The image forming portion extends not only to the periphery of the photosensitive drum but also to the transfer drum. PA1 (2) A high level of accuracy must be secured in the peripheral speeds and the gap between the photosensitive drum and the transfer drum. PA1 (3) A release mechanism is necessary in order to take out the photosensitive drum.
In the color image forming apparatus using the transfer drum described above, the mechanism gets complicated as described in items (2) and (3) and the cost of production becomes high. Moreover, image forming means are not concentrated around the photosensitive drum in the same way as in a monochromatic image forming means which does not require a transfer drum, and the merits brought about by the integration into one unit (process unit) of the image forming portions cannot be obtained sufficiently.